This invention relates to parking brakes of the type typically utilized in large vehicles such as bulldozers, etc.
Many large vehicles utilize parking brake systems distinct from the brake systems utilized in conjunction with the control of movement of ground engaging driving means, as wheels, crawler-type tracks, or the like. Because of the mass of such vehicles, the parking brake must have large capacity and, frequently, disc-type brakes of the type employing a pack of brake discs are utilized.
Such brakes may be generally categorized as pressure fluid-engaged, spring-disengaged brakes or spring-engaged, pressure fluid-disengaged brakes. In general, it is preferable to use the latter type, that is, spring-engaged, pressure fluid-disengaged brakes for the reason that when the vehicle is to be parked, in many cases, its engine will be turned off with the result that auxiliary systems such as a hydraulic pump which supplies pressure fluid to the brake will be inoperative so that no pressure fluid would be available to engage the brake for parking purposes if the brake were pressure fluid-engaged, spring-disengaged.
Another advantage of the spring-engaged, pressure fluid-disengaged type of brake is provided when there is a failure in the pressure fluid supply system, which pressure fluid frequently is utilized in performing a variety of control functions for the vehicle as well as in connection with the vehicle parking brake. In such a case, the failure will result in automatic engagement of the parking brake simultaneously with the loss of other hydraulic control functions to halt the vehicle without regard to attentiveness of the operator.
Spring-engaged, pressure fluid-disengaged parking brakes are not totally without their disadvantages, however. The principal disadvantage occurs when the vehicle is to be moved as, for example, for servicing purposes, and the pressure fluid supply system is inoperative, either due to system failure or to the inability to start the vehicle engine to drive the pressure fluid supply system. In such a case, the parking brake will be engaged, making it difficult to move the vehicle.